A spark plug that ignites an air mixture by generating a spark discharge by applying a high voltage in a discharge gap disposed between a center electrode and a ground electrode, the center electrode and the ground electrode are disposed with an insulator therebetween, for an ignition of an internal combustion engine is widely used for a spark plug for an internal combustion engine.
In recent years, reduction of a diameter of a spark plug is desired for increasing the degree of freedom of design of an engine, to attempt cooling performance improvement of the engine, or to increase the size of an intake valve for increasing an intake air amount.
A spark plug includes a ceramic insulator having a channel extending in an axial direction, a center electrode inserted into a distal end side of the pore, and a cylindrical main fitting disposed on an outer periphery of the insulator is disclosed in Patent Document 1. The insulator includes a long leg portion located at a distal end portion thereof, and a tapered portion extending from a rear end of the long leg portion towards a rear end side in the axial direction of which a diameter increases as it reaches toward the rear end side in the axial direction. The main fitting includes a stepped portion that protrudes radially inward and has an engaging surface where the tapered portion is engaged directly or indirectly, and a threaded portion located on an outer peripheral side of the stepped portion and screwed into a mounting hole of a combustion device.
Then, stress concentration to a boundary portion between the long leg portion and the tapered portion is reduced by setting a screw diameter of the threaded portion to M12 or less, and a relationship between a longitudinal area of the insulator and a longitudinal area of the main fitting at a predetermined position in predetermined range, thereby suppressing cracking of the insulator due to thermal shock from occurring.